Method of producing polarizing plate, and liquid crystal display comprising the polarizing plate

ABSTRACT

A polarizing plate produced according to the present invention includes a polarizing film and a protective layer bonded to a surface of the polarizing film, where the protective layer has no irregularities like record grooves caused by stretching of the polarizing film, so that the polarizing plate with an improved appearance provides clear images even when reflected light is applied. Such a polarizing plate is produced by laminating a protective layer on at least one surface of a polarizer while limiting moisture content of the polarizer to a range from 5% to 30%. A value for the moisture content is obtained by a calculation based on an equation of moisture content (%)=[(A−B)/B]×100, when A denotes weight of the polarizer before bonding and B denotes weight of the polarizer after being kept in a dryer of 120° C. for seven hours.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of polarizing plates for aliquid crystal display (LCD) and a liquid crystal display usingpolarizing plates.

2. Description of the Related Art

Recently, the use of LCDs in personal computers (PCs) or the like hasincreased sharply, and they have been used for monitors as well.

Typical polarizing plates used for display devices such as LCDs areproduced from polyvinyl alcohol (PVA) films or the like. The PVA filmsare dyed with dichroic iodine or dichroic dyestuff, and then crosslinkedwith boric acid, borax or the like. The films are then be stretcheduniaxially. The stretching step can be included in the dyeing and/orcrosslinking steps. Alternatively, the film can be stretched beforeand/or after the dyeing and/or crosslinking steps. In general, the filmis dried in a dryer or the like after the dyeing and crosslinking steps,and it is bonded to a protective layer such as a triacetylcellulose(TAC) film through an adhesive.

A polarizing plate used for a LCD is required to have high transmittanceand a high polarization degree. For this purpose, a PVA film should bestretched at a high ratio. However, when a PVA film is stretched at ahigh stretch ratio, irregularities like grooves on a record may begenerated on the polarizer surface, and this will impair the appearance.The irregularities can be recognized even through a protective film likea TAC film or a polyethylene terephthalate (PET) film bonded to thepolarizer. As a result, images provided by the polarizing plate will beblurred when reflected light is applied.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a polarizing platehaving an improved appearance and also a liquid crystal displaycomprising the polarizing plate. In this context, “improved appearance”means that a protective layer of the polarzing plate substantially hasno irregularities like grooves on a record caused by stretching of apolarzing film bonded to the protective layer. Such a polarizing platecan provide clear images even when reflected light is applied.

In a method of producing a polarizing plate according to the presentinvention, a protective layer is bonded to at least one surface of apolarizer so as to provide a polarizing plate having transmittance of atleast 35% and a polarization degree of at least 90%. This method ischaracterized in that moisture content of the polarizer is in a rangefrom 5% to 30% during a step that the protective layer is bonded to thepolarizer. A measurement value is obtained by a calculation based on anequation of moisture content (%)=[(A−B)/B]×100, when A denotes weight ofthe polarizer before bonding and B denotes weight of the polarizer afterbeing kept in a dryer of 120° C. for seven hours.

When the moisture content is less than 5%, the polarizer becomes hard.This causes irregularities like record grooves on the surface, and acrosslinking agent tends to be deposited from the polarizer so as todeteriorate the appearance. Moisture content over 30% will cause somedisadvantages such as adhesion failure when the polarizer is bonded to aprotective layer such as a TAC film. Moreover, irregularities occur dueto discoloring of iodine in the polarizing plate during a dryingtreatment subsequent to bonding of the TAC film. A further preferredrange for the moisture content of the polarizer is from 9% to 27%.

The surface roughness of the polarizing plate in a directionperpendicular to the stretching direction plate is at most 0.04 μm onthe basis of the centerline average roughness. When the roughnessexceeds 0.04 μm, visual recognition of irregularities may befacilitated. That is, irregularities will be prominent. It is especiallypreferable that the surface roughness is not more than 0.01 μm, since nostreaks will be recognized visually.

The surface roughness can be calculated in accordance with JapaneseIndustrial Standard (JIS) B 0601-1994.

Preferably, the polarizer is bonded to a protective layer through anadhesive layer, so that peeling of the protective layer from thepolarizer can be prevented.

The polarizer is produced by stretching a hydrophilic polymer film whiledyeing in a dye bath containing dichroic iodine or dichroic dyestuff,and then crosslinking in a crosslinking bath containing a crosslinkingagent. A preferred example of such hydrophilic polymer films is apolyvinyl alcohol-based film from the aspect of the excellentdye-affinity.

In one embodiment, the polarizing plate can be either a reflective or asemitransparent reflective polarizing plate obtained by laminatingeither a reflecting plate or a semitransparent reflecting plate on anyof the above-mentioned polarizing plates.

In one embodiment, the polarizing plate can be obtained by laminating aretardation plate (λ plate) on any of the above-mentioned polarizingplates so as to cope with either elliptically or circularly polarizedlight.

In one embodiment, the polarizing plate can be obtained by laminating aviewing angle compensating film on any of the above-mentioned polarizingplates.

In one embodiment, the polarizing plate can be obtained by laminating abrightness-enhanced film on any of the above-mentioned polarizing platesby using an adhesive or a pressure-sensitive adhesive.

A liquid crystal display according to the present invention comprises aliquid crystal cell and a polarizing plate prepared in theabove-mentioned process, and the polarizing plate is provided to atleast one surface of the liquid crystal cell.

DETAILED DESCRIPTION OF THE INVENTION

According to a method of producing a polarizing plate, a polarizerhaving a polarizing function is obtained by subjecting a PVA film torespective steps such as swelling, dyeing, stretching, crosslinking anddrying. Later, the polarizer is bonded to a protective film of TAC, PETor the like through an adhesive or a pressure-sensitive adhesive so asto provide a polarizing plate.

There is no specific limitation on the order of four steps of swelling,dyeing, stretching and crosslinking. Some or all of the four steps canbe performed simultaneously.

In the present invention, irregularities like record grooves on thepolarizer or on the polarizing plate can substantially be prevented bylimiting the moisture content of the polarizer in a range from 5% to 30%during a step of bonding the polarizer and the protective layer.

Generally, a polarizer of a PVA film, which is processed through stepsincluding swelling, dyeing, stretching, crossliking and drying, is woundinto a roll for the following steps. Such a polarizer can be produced ina continuous series of steps including adjustment of the moisturecontent of the polarizer by humidification or the like, before bondingthe polarizer to a protective layer such as a TAC film.

A typical process of producing a polarizing film comprises three stepsof dyeing, crosslinking and stretching. In a dyeing step, a PVA film isdyed in a bath containing dichroic iodine or dyestuff. In a crosslinkingstep, the film is crosslinked in a bath containing a PVA-crosslinkingagent such as boric acid and borax. In a stretching step, the PVA filmis stretched. Stretching is often performed simultaneously with thedyeing and crosslinking steps, but it can be carried out separately.Alternatively, the dyeing step and the crosslinking step can beperformed at the same time. Subsequent to the three steps, the PVA filmis dried and then, a protective layer such as a TAC film or a PET filmis bonded to at least one surface of the PVA film.

In one embodiment, a polarizer (polarizing film) is prepared from aconventional hydrophilic polymer film comprising a suitable vinylalcohol-based polymer such as polyvinyl alcohol and partially formalizedpolyvinyl alcohol. The film is treated in a suitable order and asuitable process, for example, dyeing with a dichroic substance selectedfrom, e.g., iodine and dichroic dyestuff, stretching and crosslinking. Apreferable polarizer will transmit linearly polarized light when naturallight enters. It is more preferable that the polarizer has excellentoptical transmittance and polarization degree.

A polarizing plate having high transmittance and a high polarizationdegree can comprise a polarizer (polarzing film) that is prepared bystretching a hydrophilic polymer film or the like having a thickness ina range from 10 μm to 200 μm, or preferably from 30 μm to 80 μm, in atotal stretch ratio ranging from 4 to 7 times the original length, orpreferably from 5 to 6.5 times. When the stretch ratio is less than 4,the obtained polarizer would not have a sufficient polarization degree.When the total stretch ratio exceeds 7, the film tends to break duringstretching, so that stable supply of polarizing films will be prevented.A hydrophilic polymer film having a thickness of less than 10 μm isdifficult to stretch because the film tends to break, while ahydrophilic polymer film having a thickness of more than 200 μm isdifficult to dry during film-formation and thus, problems such asfoaming will occur easily. Specifically, it is difficult to dry such athick film uniformly, and a film that is not dried uniformly may causeswelling and dye-irregularities in manufacturing the polarizing film.

Any appropriate transparent film can be used for a protective film toform a transparent protective layer on at least one surface of apolarizer (polarizing film). One typical and non-limiting example ofpolymers for the protective film is an acetate-based resin such astriacetylcellulose. Examples of alternative polymers include transparentfilms of resins based on polycarbonate, polynorbornene, polyesters suchas PET, polyether sulfone, polyamide, polyimide, polyolefins such aspolyethylene, polystyrene, and acrylic substances, or films of resinsthat will be cured by heat or ultraviolet rays, based on acrylicsubstances, urethane, acrylic urethane, epoxy, and silicones.

A transparent protective film preferred especially from the aspect ofpolarizing characteristics and durability is a TAC film having a surfacesaponified with an alkali substance or the like. Transparent protectivefilms formed on both surfaces of a polarizing film are not necessarilymade of identical polymers.

A transparent protective film used for the protective layer can betreated to provide properties such as hard coating, antireflection,anti-sticking, diffusion and anti-glaring, as long as the purposes ofthe present invention are not sacrificed. Hard coating treatment isapplied, for example, to prevent scratches on the surfaces of thepolarizing plate. A surface of the transparent protective film can beapplied with a coating film of a cured resin with excellent hardness andsmoothness, e.g., a silicone-based ultraviolet-cure type resin.

Antireflection treatment may be applied to prevent reflection of outdoordaylight on the surface of the polarizing plate. Such an anti-reflectionfilm or the like can be formed in a known method. Anti-stickingtreatment is applied to prevent adherence of adjacent layers. Anti-glaretreatment is applied to prevent visibility of light transmitted throughthe polarizing plate from being hindered by outdoor daylight reflectedon the polarizing plate surface. Anti-glare treatment can be carried outby providing microscopic asperity on a surface of a transparentprotective film in an appropriate manner, e.g., by roughening thesurface by sand-blasting or embossing, or by blending transparentparticles.

The above-mentioned transparent fine particles will be selected fromsilica, alumina, titania, zirconia, stannic oxide, indium oxide, cadmiumoxide, antimony oxide or the like, and the particles have an averagediameter ranging from 0.5 μm to 20 μm. Inorganic fine particles havingelectroconductivity can be used as well. Alternatively, the particlescan be organic fine particles comprising, for example, crosslinked oruncrosslinked polymer particles. An amount of the transparent fineparticles ranges from 2 weight parts to 70 weight parts, and generally,from 5 weight parts to 50 weight parts, for 100 weight parts of atransparent resin.

An anti-glare layer comprising transparent fine particles can beprovided as the transparent protective layer or a coating layer appliedonto a transparent protective layer surface. The anti-glare layer canfunction as a diffusion layer to diffuse light transmitted through thepolarizing plate in order to enlarge visual angles (this function isdenoted as visual angle compensation). The above-mentioned layers suchas the antireflection layer, the anti-sticking layer, the diffusionlayer and the anti-glare layer can be provided as an sheet of opticallayers comprising these layers separately from the transparentprotective layer.

There is no specific limitation on a method to adhere the polarizer(polarizing film) and the transparent protective film. Adhesion can beapplied, for example, by using adhesives such as an adhesive comprisingvinyl alcohol-based polymer, or an adhesive comprising at least thevinyl alcohol-based polymer and a water-soluble agent to crosslink thevinyl alcohol-based polymer, such as boric acid, borax, glutaraldehyde,melamine and oxalic acid. A polyvinyl alcohol-based adhesive ispreferred especially since it has the best adherence with polyvinylalcohol-based films. Such an adhesive layer is formed by, for example,applying and drying an aqueous solution, and an additive or a catalystsuch as an acid can be blended in preparation of the aqueous solution ifrequired.

A polarizing plate of the present invention can be laminated withanother optical layer in order to be used as an optical member. Thoughthere is no specific limitation on the optical layer, one or moresuitable optical layer applicable for formation of a liquid crystaldisplay can be used, and the optical layer can be selected from, forexample, a reflecting plate, a semitransparent reflecting plate, aretardation plate such as a λ plate like a half wavelength plate and aquarter wavelength plate, a viewing angle compensating film, and abrightness-enhanced film. In a preferred embodiment, a reflectivepolarizing plate or a semitransparent reflective polarizing plate formedby laminating an additional reflecting plate or a semitransparentreflecting plate on the above-mentioned polarizing plate comprising apolarizer and a protective layer according to the present invention; apolarizing plate formed by laminating an additional retardation plate onthe above-mentioned polarizing plate comprising a polarizer and aprotective layer; a polarizing plate having a viewing angle compensatingfilm laminated additionally on the above-mentioned polarizing platecomprising a polarizer and a protective layer; and a polarizing platehaving a brightness-enhanced film laminated additionally on theabove-mentioned polarizing plate comprising a polarizer and a protectivelayer is used.

A reflecting plate is provided to a polarizing plate in order to form areflective polarizing plate. In general, such a reflective polarizingplate is arranged on a backside of a liquid crystal cell in order tomake a liquid crystal display to reflect incident light from a visibleside (display side). The reflective polarizing plate has some merits,for example, assembling of light sources such as backlight can beomitted, and the liquid crystal display can be thinned further.

The reflective polarizing plate can be formed in an appropriate mannersuch as attaching a reflecting layer of metal or the like on one surfaceof the polarizing plate. For example, a transparent protective film isprepared by matting one of the surfaces if required. On this surface, afoil comprising a reflective metal such as aluminum or a deposition filmis applied to form a reflecting layer.

An additional example of a reflective polarizing plate comprises theabove-mentioned transparent protective film having a surface of amicroscopic asperity due to contained fine particles, and also areflecting layer corresponding to the microscopic asperity. Thereflecting layer having a microscopic asperity surface diffuses incidentlight irregularly so that directivity and glare can be prevented andirregularity in color tones can be controlled. This transparentprotective film can be formed by attaching a metal directly on a surfaceof a transparent protective film in any appropriate methods includingdeposition such as vacuum deposition, and plating such as ion platingand sputtering.

Alternatively, the reflecting plate can be used as a reflecting sheetformed by providing a reflecting layer onto a proper film similar to thetransparent protective film. Since a typical reflecting layer of areflecting plate is made of a metal, it is used preferably in a statecoated with a film, a polarizing plate or the like in order to preventthe reflection rate from reduction due to oxidation. As a result, theinitial reflection rate is maintained for a long period, and a separateprotective layer can be omitted.

A semitransparent polarizing plate is provided by replacing thereflecting layer in the above-mentioned reflective polarizing plate by asemitransparent reflecting layer, and it is exemplified by a half mirrorthat reflects and transmits light at the reflecting layer. In general,such a semitransparent polarizing plate is arranged on a backside of aliquid crystal cell. In a liquid crystal display comprising thesemitransparent polarizing plate, incident light from the visible side(display side) is reflected to display an image when a liquid crystaldisplay is used in a relatively bright atmosphere, while in a relativelydark atmosphere, an image is displayed by using a built-in light sourcesuch as a backlight in the backside of the semitransparent polarizingplate. In other words, the semitransparent polarizing plate can be usedto form a liquid crystal display that can save energy for a light sourcesuch as a backlight under a bright atmosphere, while a built-in lightsource can be used under a relatively dark atmosphere.

The above-mentioned polarizing plate comprising a polarizer and aprotective layer can have an additional Laminate of a retardation plate.

The retardation plate is used for modifying linearly polarized light toeither elliptically polarized light or circularly polarized light,modifying either elliptically polarized light or circularly polarizedlight to linearly polarized light, or modifying a polarization directionof linearly polarized light. For example, a retardation plate called aquarter wavelength plate (λ/4 plate) is used for modifying linearlypolarized light to either elliptically polarized light or circularlypolarized light, and for modifying either elliptically polarized lightor circularly polarized light to linearly polarized light. A half wavelength plate (λ/2 plate) is used in general for modifying a polarizationdirection of linearly polarized light.

The above-described polarizing plate concerning elliptical polarizedlight is effective in compensating (preventing) colors (blue or yellow)generated due to birefringence in a liquid crystal layer of a supertwist nematic (STN) liquid crystal display so as to provide ablack-and-white display free of such colors. Controllingthree-dimensional refractive index is preferred further since it cancompensate prevent) colors that will be observed when looking a screenof the liquid crystal display from an oblique direction. A polarizingplate concerning circularly polarized light is effective in adjustingcolor tones of an image of a reflective liquid crystal display that hasa color image display, and the polarizing plate serves to preventreflection as well.

Specific examples of the retardation plates include birefringent films,oriented films of liquid crystal polymers, sheets comprising film andoriented layers supported by the films, and incline-oriented films. Thebirefringent films can be prepared by stretching films of any suitableliquid crystal polymers such as polycarbonate, polyvinyl alcoholpolystyrene, polymethyl methacrylate, polyolefins includingpolypropylene, polyalylate, and polyamide. An incline-oriented film isproduced, for example, by bonding a heat shrinkable film onto a polymerfilm and stretching and/or shrinking the polymer film under an influenceof the shrinking force provided by heat, or by orienting obliquely aliquid crystal polymer.

A polarizing plate described below comprises the above-mentionedpolarizer and protective layer, and further an additional viewing anglecompensating film laminated on the polarizing plate.

A viewing angle compensating film is used for widen an visual angle sothat an image can be clear relatively when a screen of a liquid crystaldisplay is seen not in a direction perpendicular to the screen but in aslightly oblique direction.

Such a viewing angle compensating film can be a triacetylcellulose filmcoated with a discotic liquid crystal, or a retardation plate. While anordinary retardation plate is a birefringent polymer film that isstretched uniaxially in the face direction, a retardation plate used foran viewing angle compensating film is a two-way stretched film such as abirefringent polymer film stretched biaxially in the face direction andan incline-oriented polymer film with controlled birefringence in thethickness direction that is stretched uniaxially in the face directionand stretched also in the thickness direction. The incline-oriented filmis prepared by, for example, bonding a heat shrinkable film to a polymerfilm and stretching and/or shrinking the polymer film under an influenceof shrinkage force provided by heat, or by orienting obliquely a liquidcrystal polymer. A polymer as a material of the retardation plate issimilar to the polymer used for the above-mentioned retardation plate.

A polarizing plate described below is produced by laminating abrightness-enhanced film additionally on the above-mentioned polarizingplate comprising a polarizer and a protective layer. Generally, thispolarizing plate is arranged on a backside of a liquid crystal cell.When natural light enters, by reflection from a backlight or a backsideof a liquid crystal display etc., the brightness-enhanced film reflectslinearly polarized light of a predetermined polarizing axis orcircularly polarized light in a predetermined direction while the samefilm transmits other light. It allows entrance of light from a lightsource such as a backlight so as to obtain transmitted light in apredetermined polarization state, while reflecting light other thanlight in the predetermined polarization state. Light that is reflectedat this brightness-enhanced film is reversed through a reflecting layeror the like arranged additionally behind the brightness-enhanced film.The reversed light that re-enters the brightness-enhanced plate istransmitted partly or entirely as light in a predetermined polarizationstate, so that light transmitting the brightness-enhanced film isincreased and polarized light that is hardly absorbed in the polarizeris supplied. As a result, quantity of light available for the liquidcrystal display etc. can be increased to enhance brightness. When lightenters through a polarizer from the backside of a liquid crystal cell byusing a backlight or the like without using any brightness-enhancedfilms, most light is absorbed in the polarizer but not transmitted thepolarizer if the light has a polarization direction inconsistent withthe polarization axis of the polarizer. Depending on characteristics ofthe polarizer, about 50% of light is absorbed in the polarizer, and thisdecreases quantity of light available in the liquid crystal display orthe like and makes the image dark. The brightness-enhanced filmrepeatedly prevents light having a polarization direction to be absorbedin the polarizer from entering the polarizer, and reflects the light onthe brightness-enhanced film, reverses the light through a reflectinglayer or the like arranged behind, and makes the light re-enter thebrightness-enhanced plate. Since the polarized light that is reflectedand reversed between them is transmitted only if the light has apolarization direction to pass the polarizer, light from a backlight orthe like can be used efficiently for displaying images of a liquidcrystal display in order to provide a bright screen.

A suitable example of the brightness-enhanced film is selected from amultilayer thin film of a dielectric or a multilayer lamination of thinfilms with varied refraction aeolotropy (e.g., “D-BEF” supplied by 3MCo.) that transmits linearly polarized light having a predeterminedpolarization axis while reflecting other light, and a cholesteric liquidcrystal layer, more specifically, an oriented film of a cholestericliquid crystal polymer or an oriented liquid crystal layer fixed onto asupportive substrate (e.g., “PCF 350” supplied by Nitto DenkoCorporation; “Transmax” supplied by Merck and Co., Inc.) that reflectseither clockwise or counterclockwise circularly polarized light whiletransmitting other light.

Therefore, for a brightness-enhanced film to transmit linearly polarizedlight having a predetermined polarization axis, the transmission lightenters the polarizing plate by matching the polarization axis so thatabsorption loss due to the polarizing plate is controlled and the lightcan be transmitted efficiently. For a brightness-enhanced film totransmit circularly polarized light, i.e., a cholesteric liquid crystallayer, preferably, the transmission circularly polarized light isconverted to linearly polarized light before entering the polarizingplate in an aspect of controlling of the absorption loss, though thecircularly polarized light can enter the polarizer directly. Circularlypolarized light can be converted to linearly polarized light by using aquarter wavelength plate for a retardation plate.

A retardation plate having a function as a quarter wavelength plate in awide wave range including a visible light region can be obtained, forexample, by overlapping a retardation layer functioning as a quarterwavelength plate for monochromatic light such as light having 550 nmwavelength and another retardation plate showing a separate opticalretardation property (e.g., a retardation plate functioning as a halfwavelength plate). Therefore, a retardation plate arranged between apolarizing plate and a brightness-enhanced film can comprise a singlelayer or at least two layers of retardation layers.

A cholesteric liquid crystal layer also can be provided by combininglayers different in the reflection wavelength and it can be configuredby overlapping two or at least three layers. As a result, the obtainedretardation plate can reflect circularly polarized light in a widewavelength range including a visible light region, and this can providetransmission circularly polarized light in a wide wavelength range.

A polarizing plate according to the present invention can be made bylaminating a polarizing plate and two or at least three optical layers,similarly to the above-described polarization-separation type polarizingplates. In other words, the polarizing plate can be a reflectivepolarizing plate or a semitransparent polarizing plate for ellipticallypolarized light, which is prepared by combining either theabove-mentioned reflective polarizing plate or a semitransparentpolarizing plate with a retardation plate. An optical member comprisinga lamination of two or at least three optical layers can be formed in amethod of laminating layers separately in a certain order formanufacturing a liquid crystal display etc. Since an optical member thathas been laminated previously has excellent stability in quality andassembling operability, efficiency in manufacturing a liquid crystaldisplay can be improved. Any appropriate adhesion means such as apressure-sensitive adhesive can be used for laminating the polarizingplate and optical layers.

A pressure-sensitive adhesive layer can be provided to a polarizingplate or to an optical member in the present invention for adhesion withother members such as a liquid crystal cell. The pressure-sensitiveadhesive layer can contain any suitable pressure-sensitive adhesivessuch as an acrylic adhesive in accordance with conventional techniques.Particularly, pressure-sensitive adhesive layers having a low moistureabsorption coefficient and an excellent heat resistance is preferredfrom the aspect of prevention of foaming or peeling caused by moistureabsorption or prevention of decrease in the optical properties andwarping of a liquid crystal cell caused by difference in thermalexpansion coefficients. As a result, a high quality liquid crystaldisplay having excellent durability can be produced. Thepressure-sensitive adhesive layer can include fine particles to obtainoptical diffusivity. Pressure-sensitive adhesive layers can be providedto appropriate surfaces if required. For example, a polarizing platecomprising a polarizer and a protective layer can be provided with apressure-sensitive adhesive layer on at least one surface of theprotective layer.

When a pressure-sensitive adhesive layer is exposed on a surface of thepolarizing plate or the optical member, preferably, thepressure-sensitive adhesive layer is covered with a separator by thetime the pressure-sensitive adhesive layer is used so that contaminationwill be prevented. The separator can be made of an appropriate thinsheet by coating a peeling agent if required, and the peeling agent maybe selected, for example, from a silicone-based agent, a long-chainalkyl-based agent, a fluorine-based agent, an agent comprisingmolybdenum sulfide or the like.

The above-described members composing a polarizing plate and an opticalmember, such as a polarizer, a transparent protective film, an opticallayer and a pressure-sensitive adhesive layer, can have ultravioletabsorption power as a result of treatment with an ultraviolet absorbersuch as an ester salicylate compound, a benzophenone compound, abenzotriazole compound, a cyanoacrylate compound, and a nickel complexsalt compound.

Polarizing plates according to the present invention can be usedpreferably for forming various devices such as LCDs. Such a polarizingplate is arranged on at least one surface of a liquid crystal cell inorder to form various devices such as a liquid crystal display. Theliquid crystal display is selected from devices of conventionally knownstructures, such as transmission type, reflection type, or atransmission-reflection type. A liquid crystal cell to compose theliquid crystal display can be selected from appropriate cells of such asactive matrix driving type represented by a thin film transistor, asimple matrix driving type represented by a twist nematic type and asuper twist nematic type.

When polarizing plates or optical members are arranged on both surfacesof a liquid crystal cell, the polarizing plates or the optical memberson the surfaces can be the same or can be varied. Moreover, for forminga liquid crystal display, one or at least two layers of appropriatemembers such as a prism array sheet, a lens array sheet, an opticaldiffuser and a backlight can be arranged at proper positions.

The present invention will be described below more specifically byreferring to Examples and Comparative Examples.

EXAMPLE 1

A polarizer was obtained by dyeing a PVA film supplied by Kuraray Co.,Ltd. (9X75RS, having a polymerization degree of 2400 and a thickness of75 μm) in a first bath (a 30° C. aqueous solution containing both iodineand potassium iodine (KI)) while stretching to 3 times, furtherstretching in a second bath (a 55° C. aqueous solution containing bothboric acid and KI) so as to stretch the film to 6 times in total itsoriginal length. Later, the polarizer was adjusted to have moisturecontent of 6% by means of a dryer and a humidifier under controlledcondition for temperature, humidity, air volume, and time. Subsequently,TAC films were bonded to a top and a bottom surface of the polarizerthrough a PVA-based adhesive in order to provide a polarizing plate. Avalue for the moisture content is obtained by a calculation based on anequation of moisture content (%)=[(A−B)/B]×100, when A denotes weight ofthe polarizer before bonding and B denotes weight of the polarizer afterbeing kept in a dryer of 120° C. for seven hours.

EXAMPLE 2

A polarizer was obtained by dyeing a PVA film supplied by Kuraray Co.,Ltd. (9X75RS) in a first bath (a 30° C. aqueous solution containing bothiodine and KI) while stretching to 3 times, further stretching in asecond bath (a 55° C. aqueous solution containing both boric acid andKI) so as to stretch the film to 6 times in total. Later, the polarizerwas adjusted to have moisture content of 15% by means of a dryer and ahumidifier under controlled conditions for temperature, humidity airvolume, and time. Subsequently, TAC films were bonded to both surfacesof the polarizer through a PVA-based adhesive in order to provide apolarizing plate.

EXAMPLE 3

A polarizer was obtained by dyeing a PVA film supplied by Kuraray Co.,Ltd. (9X75RS) in a first bath (a 30° C. aqueous solution containing bothiodine and KI) while stretching to 3 times, further stretching in asecond bath (a 55° C. aqueous solution containing both boric acid andKI) so as to stretch the film to 6 times in total. Later, the polarizerwas adjusted to have moisture content of 26% by means of a dryer and ahumidifier under controlled conditions for temperature, humidity, airvolume, and time. Subsequently, TAC films were bonded to both surfacesof the polarizer through a PVA-based adhesive in order to provide apolarizing plate.

Comparative Example 1

A polarizer was obtained by dyeing a PVA film supplied by Kuraray Co.,Ltd. (9X75RS) in a first bath (a 30° C. aqueous solution containing bothiodine and KI) while stretching to 3 times, further stretching in asecond bath (a 55° C. aqueous solution containing both boric acid andKI) so as to stretch the film to 6 times in total. Later, the polarizerwas adjusted to have moisture content of 4% by means of a dryer and ahumidifier under controlled conditions for temperature, humidity, airvolume, and time. Subsequently, TAC films were bonded to both surfacesof the polarizer through a PVA-based adhesive in order to provide apolarizing plate.

Comparative Example 2

A polarizer was obtained by dyeing a PVA film supplied by Kuraray Co.,Ltd. (9X75RS) in a first bath (a 30° C. aqueous solution containing bothiodine and KI) while stretching to 3 times, further stretching in asecond bath (a 55° C. aqueous solution containing both boric acid andKI) so as to stretch the film to 6 times in total. Later, the polarizerwas adjusted to have moisture content of 35% by means of a dryer and ahumidifier under controlled conditions for temperature, humidity, airvolume, and time. Subsequently, TAC films were bonded to both surfacesof the polarizer through a PVA-based adhesive in order to provide apolarizing plate. In Comparative Example 2, however, irregularitiesoccurred in the surfaces of the polarizing plate due to the dryingtreatment for bonding to the TAC film, because the moisture content wasas high as 35%.

The polarizing plates obtained in the Examples 1-3 and ComparativeExamples 1-2 were evaluated. Optical properties to be measured weretransmittance and a polarization degree for each plate. An instrumentfor measuring surface roughness shape (SURFCOM 470A supplied by TOKYOSEIMITSU CO., LTD.) was used for measuring centerline average roughness(Ra) in a direction of the polarizing axis of the polarizing plate (adirection perpendicular to the stretching axis) and also mean spacing(Sm) of irregularities forming the streaks. Streaks were checkedvisually for the polarizing plates. The results are shown in Table 1.

TABLE 1 Polarizer Polarizing plate Moisture Transmittance PolarizatonSurface roughness content (%) (%) degree (%) Ra (μm) Sm (mm) Visualobservation Com. Ex. 1 4 43.8 99.95 0.08 0.75 Prominent streaks Example1 6 43.8 99.95 0.03 1.81 Pale streaks Example 2 15 43.8 99.94 0.01 orless Unmeasurable No streak Example 3 26 43.8 99.94 0.01 or lessUnmeasurable No streak Com. Ex 35 43.8 99.90 0.01 or less UnmeasurableNo streak *Com. Ex.: Comparative Example

As indicated in Table 1, values of the centerline average roughness (Ra)for the polarizing plates (Examples 2-3) of the present invention weresmall, and the mean spacing of the irregularities (Sm) was unmeasurable.No streaks were recognized visually. Pale streaks recognized in thepolarizing plate of Example 1 were not a substantial obstacle in use.

For the polarizing plate of Comparative Example 1, which was producedunder a condition of humidity out of the claimed range, the centerlineaverage roughness (Ra) was remarkable and the mean spacing (Sm) waslarge. Moreover, streaks were recognized visually. The polarizing platein Comparative Example 2 was good for the centerline average roughness(Ra) and the mean spacing (Sm). However, the polarization degree wasinferior, and irregularities in the surface were recognized, resultingin inferior appearance.

As mentioned above, the present invention provides a polarizing platehaving improved appearance and also a liquid crystal display comprisingthe polarizing plate. In a method for producing the polarizing plate bybonding a protective layer on at least one surface of a polarizer,moisture content of the polarizer is limited to a range from 5% to 30%so that the protective layer would not have substantial irregularitieslike record grooves on the surface, which would be caused by stretchingof the polarizing film. Such a polarizing plate can provide clear imageseven when reflected light is applied. Moreover, the method of thepresent invention can provide a polarizing plate having transmittance ofat least 35% and a polarization degree of at least 90%.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, all changesthat come within the meaning and range of equivalency of the claims areintended to be embraced therein.

1. A method of producing a polarizing plate comprising a polarizer and aprotective layer bonded to at least one surface of the polarizer,wherein moisture content of the polarizer is in a range from 5% to 30%when the protective layer is bonded to the polarizer.
 2. The methodaccording to claim 1, wherein surface roughness of the polarizing platein a direction perpendicular to the stretching direction is 0.04 μm orless on the basis of the centerline average roughness.
 3. The methodaccording to claim 1, wherein the protective layer is bonded to thepolarizer through an adhesive layer.
 4. The method according to claim 1,wherein the polarizer is prepared by stretching a hydrophilic polymerfilm while dyeing the hydrophilic polymer film in a dye bath containinga dye selected from the group consisting of dichroic iodine and dichroicdyestuff and crosslinking in a crosslinking bath containing acrosslinking agent.
 5. The method according to claim 4, wherein thehydrophilic polymer film is a polyvinyl alcohol-based film.
 6. Themethod according to claim 1, wherein a reflecting plate is additionallylaminated.
 7. The method according to claim 1, wherein a semitransparentreflecting plate is additionally laminated.
 8. The method according toclaim 1, wherein a retardation plate (λ plate) is additionally laminatedin order to cope with elliptically or circularly polarized light.
 9. Themethod according to claim 1, wherein a viewing angle compensating plateis additionally laminated.
 10. The method according to claim 1, whereina brightness enhanced film is additionally laminated through either anadhesive or a pressure-sensitive adhesive.
 11. A liquid crystal displaycomprising a liquid crystal cell and a polarizing plate bonded to atleast one surface of the liquid crystal cell, wherein the polarizingplate comprises a protective layer bonded to at least one surface of apolarizer when moisture content of the polarizer is in a range from 5%to 30%.
 12. The liquid crystal display according to claim 11, whereinsurface roughness of the polarizing plate in a direction perpendicularto the stretching direction is 0.04 μm or less on the basis of thecenterline average roughness.
 13. The liquid crystal display accordingto claim 11, wherein the protective layer is bonded to the polarizerthrough an adhesive layer.
 14. The liquid crystal display according toclaim 11, wherein the polarizer is prepared by stretching a hydrophilicpolymer film while dyeing the hydrophilic polymer film in a dye bathcontaining a dye selected from the group consisting of dichroic iodineand dichroic dyestuff and crosslinking in a crosslinking bath containinga crosslinking agent.
 15. The liquid crystal display according to claim14, wherein the hydrophilic polymer film is a polyvinyl alcohol-basedfilm.
 16. The liquid crystal display according to claim 11, wherein areflecting plate is additionally laminated.
 17. The liquid crystaldisplay according to claim 11, wherein a semitransparent reflectingplate is additionally laminated.
 18. The liquid crystal displayaccording to claim 11, wherein a retardation plate (λ plate) isadditionally laminated in order to cope with elliptically or circularlypolarized light.
 19. The liquid crystal display according to claim 11,wherein a viewing angle compensating plate is additionally laminated.20. The liquid crystal display according to claim 11, wherein abrightness enhancement film is additionally laminated through either anadhesive or a pressure-sensitive adhesive.
 21. The method according toclaim 1, wherein surface roughness of the polarizing plate in adirection perpendicular to the stretching direction is 0.03 μm or lesson the basis of the centerline average roughness.
 22. The methodaccording to claim 1, wherein surface roughness of the polarizing platein a direction perpendicular to the stretching direction is 0.03 μm orless on the basis of the centerline average roughness.
 23. The liquidcrystal display according to claim 11, wherein surface roughness of thepolarizing plate in a direction perpendicular to the stretchingdirection is 0.03 μm or less on the basis of the centerline averageroughness.
 24. The liquid crystal display according to claim 11, whereinsurface roughness of the polarizing plate in a direction perpendicularto the stretching direction is 0.01 μm or less on the basis of thecenterline average roughness.